A transistor formed with a semiconductor thin film that is formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is referred to as a thin film transistor (TFT). A TFT is an embodiment of semiconductor elements and formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. TFTs formed using amorphous silicon have a low electric field mobility but can respond to increase in size of glass substrates. On the other hand, TFTs formed using polycrystalline silicon have a high electric field mobility but need a crystallization step such as laser annealing and are not always suitable for increase in size of glass substrates.
Thus, a technique in which a TFT is formed using an oxide semiconductor as a semiconductor material and applied to an electronic device or an optical device has attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used for a switching element or the like in an image display device.
A TFT in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have a higher electric field mobility than a TFT using amorphous silicon. An oxide semiconductor film can be formed at a relatively low temperature by a sputtering method or the like, and a manufacturing process of the TFT using an oxide semiconductor is simpler than that of the TFT using polycrystalline silicon.
TFTs which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.